PHILADELPHIA — Nick Foles has been such a difference-maker for the Eagles that it begs the question: Why wasn’t he their starting quarterback from Day 1?

Chip Kelly brought his reputation as a rebel and a read-option innovator to the NFL, but it took yet another injury to broken-down Michael Vick for Philadelphia’s first-year coach to turn his team over to Foles — who has promptly done nothing less than save the Birds’ entire season.

Can Foles’ hot streak continue Sunday afternoon against the NFC North-leading Lions at Lincoln Financial Field? With the way he is playing, anything less than a mistake-free showing would be shocking.

Since returning from a concussion a month ago and taking over an Eagles team that was listing badly at 3-5, Foles has led Philly to four consecutive victories and a tie with the Cowboys for first place in the NFC East with an individual performance nothing short of astonishing.

In that span, the second-year pro from Arizona has completed 68 percent of his passes for 1,169 yards and 13 touchdowns without an interception, compiling a sparkling 144.2 passer rating in the process.

Not only that, but Foles’ season line of 19 TD passes against zero interceptions is one short of Peyton Manning’s NFL record for most TD passes without a pick to start a season, and the 152.8 passer rating that earned Foles NFC Offensive Player of the Month in November was the highest ever by an NFL quarterback in a calendar month.

This is how precise Foles has been: If Foles threw an interception on each of his next 50 pass attempts, he still would have a higher passer rating than Jets rookie Geno Smith.

The chances of Foles throwing his first pick on Sunday appear low, considering the Lions are 26th in the league in passing defense and have forced a turnover in just two of their past six games.

Foles has been such a revelation during the winning streak (he is 5-1 overall this season) that Kelly last week jokingly named him the Eagles’ starter “for the next thousand years.”

That was in sharp contrast to Kelly’s stance coming out of the preseason, when Foles lost the contest to Vick for the starting job despite Vick’s well-chronicled injury tendencies and the promise Foles had shown as a rookie under former coach Andy Reid last season.

Foles lost out because he is more of a prototypical pocket passer at 6-feet-5, 243 pounds, and not the smaller, more mobile quarterback like Vick, which Kelly considers ideal for his up-tempo option attack.

But Foles has shown that appearances can be deceiving. Foles will never be Vick, of course, but he’s shown surprising mobility during the streak and is averaging nine rushing attempts per game over the past three weeks.

The accurate and shockingly mistake-free passing is also a bit of a surprise, considering Foles threw five interceptions in 265 attempts as a rookie but hasn’t had a single pass picked off in 196 attempts this season.

That precision appears to have come from Kelly tightening his young quarterback’s workload. Reid had Foles average a whopping 38 passes in the West Coast offense last year, but Foles is averaging just 28 attempts per start in Kelly’s system this season.

As a result, all arrows are pointing up for the Eagles. Kelly’s reputation, which was hurting when Philly was 3-5, has been salvaged, and his team might have found a true franchise quarterback.